The present invention relates to a roller blind unit having a blind (such as one made up of slats which are joined together) that is wound and unwound from a take-up shaft around which it is wrapped in the form of a spiral coil.
In the case of known roller blinds as used in window openings or the like, the blind is drawn into the shut position by a pulling force acting on one end of the blind. If the blind guides are upright or at a steep enough slope, this force may be that of gravity. However, in many cases the slope may not be so steep or the blind may even be level and friction forces are likely to be greater than the force (if any) acting on the blind in the shutting direction so that an external driving force is needed for operation. Such an external force may for example be a pulling force acting on the outermost slat of the blind. However the slats of a blind are normally jointed together loosely in such a way that on pulling on the blind the joints between the slats are opened out somewhat by any such externally acting force. The blind will then not be completely light-proof and furthermore the thermal insulation effect will be impaired because of the flow of air through the blind into and out of the room on the inside of the blind and there will be air circulation. A further shortcoming with prior art roller blinds is that the outermost slat (i.e. the slat that is furthest from the take-up shaft of the blind) has the full opening force acting on it and for this reason has to be made oversize, this being true as well for its joint and the slats next to the outermost slat which have to transmit high tensile forces. In addition to this particular of roller blind design, power blind operating systems make the mechanical design more complex in other respects.